1. Field of the Invention
The present invention relates to a technology for driving a liquid-crystal optical-modulation device.
2. Description of the Related Art
In recent years, an optical-modulation device including a liquid crystal (hereinafter, “liquid-crystal optical-modulation device) has been attracting much attention. The liquid-crystal optical-modulation device is an effective device to be used in a variable optical attenuator (VOA). A driving method of the liquid-crystal optical-modulation device is similar to that of a liquid-crystal display. As the driving method of the liquid crystal display, a static driving method and a matrix driving method are used.
In the static driving method, as shown in FIG. 1, a voltage is applied to each pixel by a common signal 1 and a segment signal 2. In the matrix driving method, as shown in FIG. 2, plural pixels 4 are driven by a pulse signal 5 in the time-division manner. The matrix driving method is suitable for a two-dimensional display device such as a liquid-crystal display. However, the matrix driving method is rarely applied to the liquid-crystal optical-modulation device due to various restrictions.
To simplify the structure of a driving circuit, a drive signal to be applied to the liquid crystal should be a pulse signal. For liquid crystals, a pulse height modulation (PHM) and a pulse width modulation (PWM) are applicable. In consideration of a structure and a performance of the liquid-crystal optical-modulation device, the static driving method applying the pulse width modulation is suitable for the liquid-crystal optical-modulation device. With the pulse width modulation, a digital control can be easily performed, and a circuit size and power consumption are smaller than the pulse height modulation.
A technique of the static driving with the pulse width modulation is disclosed in, for example, Japanese Patent Application Laid-Open No. S59-157572. In the technique, a common signal having binary voltage levels and a segment signal having the same voltage levels as the common signal are applied to a common electrode and a segment electrode arranged opposite to each other across a pixel. Thus, a drive signal having ternary voltage levels is applied to a liquid crystal. FIG. 3 illustrates waveforms of a common signal C, a segment signal S, and a drive signal P in this driving technique.
As shown in FIG. 3, the drive signal P to be applied to the liquid crystal has a positive polarity in a period T1 in which an alternating electric field has one polarity, and has a negative polarity in a period T2 in which the alternating electric field has another polarity. If a direct-current component is included in the drive voltage, the life of the liquid crystal is shortened. In this driving method, an effective voltage to be applied to the liquid crystal of each pixel changes according to a pulse width of a corresponding segment signal, thereby changing a phase of light passing through the liquid crystal.
However, the driving method shown in FIG. 3 produces a non-zero period in which the applied voltage is not zero and a zero period in which the applied voltage is zero. A long zero period causes a change of the condition of the liquid crystal and a decrease of the phase shift amount of the light. In the next non-zero period, however, the phase shift amount increases. Thus, a phenomenon (waveform response) that decreases and increases the phase shift amount is repeated.
In addition, the conventional driving method shown in FIG. 3 has a problem in which resolution becomes insufficient in a range in which a characteristic of the liquid-crystal optical-modulation device significantly changes. FIG. 4 illustrates a relation between the applied voltage (effective voltage) and the phase of light passing through the liquid crystal. As shown in FIG. 4, a characteristic curve of the liquid-crystal optical-modulation device is not linear and includes an abrupt change in the slope. In the characteristic curve shown in FIG. 4, when effective voltages of the driving voltage to be applied to the liquid crystal are VA, VB, VC, and VD, phases of light are φA, φB, φC, and φD, respectively.
In an example shown in FIG. 4, the slope of the characteristic curve between φB and φC is larger than that between φA and φB. A pulse width generated in the pulse width modulation performed by a digital circuit is constant throughout the characteristic curve. Therefore, the phase shift amount corresponding to one pulse width in a range in which the slope is large represents the minimum resolution of the liquid-crystal optical-modulation device.
FIG. 5 illustrates waveforms of drive signals used in the pulse width modulation to obtain effective voltages VA, VB, VC, and VD. A drive signal PA corresponds to the effective voltage VA, and with the effective voltage VA, the phase shift amount becomes the minimum. A drive signal PD corresponds to the effective voltage VD, and with the effective voltage VD, the phase shift amount becomes the maximum.
A drive signal PC corresponds to the effective voltage VC, which corresponds to the range in which the slope of the characteristic curve is the largest. A drive signal PB corresponds to the effective voltage VB, and has a pulse width one minimum width smaller than a pulse width of the drive signal PC as shown in FIG. 5. It is impossible to generate a pulse having a pulse width between pulse widths of the drive signal PB and the drive signal PC. In other words, it is impossible to generate a voltage between the effective voltages VB and VC in the characteristic curve shown in FIG. 4. Therefore, the minimum resolution of the liquid-crystal optical-modulation device is a phase difference between φB and φC. To obtain higher resolution, a large scale circuit and a high clock frequency are necessary, thereby increasing power consumption.
Furthermore, characteristics of liquid crystal change due to a factor such as a temperature change. In the pulse width modulation, influence of such factor is calculated in advance to be reflected in modulation data. Thus, a variation due to the characteristic change is corrected to finely adjust the pulse width. However, such calculation is very complicated, and a real-time calculation is difficult to be achieved with a simple system.
With such reason, in a general liquid-crystal driving device, the variation due to the characteristic change is corrected by controlling amplitude of a drive signal. However, if this correction method is applied to the conventional driving method shown in FIG. 3, a voltage exceeds a withstand voltage of a segment driving circuit formed in an integrated circuit (IC), or power consumption increases, because the common signal and the segment signal have the same amplitude.